1883.] On the Dependence of Radiation on Temperature. 167 



who, employing Dulong and Petit's empirical formula, deduce the 

 values 1461° and 1398° C. for the solar temperature. Between these 

 extreme estimates we have those of Dr. Spoerer, 27,000° C, of 

 Zoellner, 27,700°, Professor James Dewar (1872), 16,000°, Hosetti 

 (1878), 9000°, and Hirn (1882), 20,000°. 



In my own investigations on this subject, by comparing the 

 spectrum of the sun as regards the proportion of luminous rays with 

 those of the electric arc and gas names, I have arrived at the 

 conclusion that the temperature of the photosphere does not exceed 

 2800° C, which is in close agreement with the limit assigned by M_ 

 Sainte- Claire Deville, deduced from the observations of Frankland 

 and Lockyer on the hydrogen lines in the solar spectrum. Sir William 

 Thomson, in a paper communicated to the Philosophical Society of 

 Glasgow (1882), has compared the power of the sun's radiation per 

 unit of surface with that of a Swan incandescent carbon filament, and 

 has shown that it is about sixty-seven times greater ; he concludes from 

 these data that the estimate I had formed of the solar temperature, 

 i.e., nearly 3000° C, cannot be very far from the true value. 



These diverse and indirect results have long impressed me with the 

 need of further experimental investigation of the dependence of radia- 

 tion on temperature ; and it has occurred to me lately, that the diffi- 

 culties with which Dulong and Petit had to contend in making their 

 measurements by means of a mercurial thermometer, where the losses 

 due to conduction and convection are very great, and exceedingly 

 difficult to determine, might be avoided in adopting a method of 

 conducting the experiment which forms the principal subject of my 

 present communication. 



It is well known that the measurement of electrical currents and 

 resistance is susceptible of very great accuracy compared with all 

 thermal measurements ; hence my endeavour has been to estimate 

 thermal effects entirely by electrical methods. In the Bakerian 

 Lecture for 1871, which I had the honour of delivering before the 

 Royal Society ("Proc. Hoy. Soc," vol. 19, p. 443), I showed that the 

 resistance of a platinum wire can be expressed as a linear function 

 of its temperature by an empirical formula, the constants of which 

 must be determined for each individual wire ; hence conversely, if 

 resistance of a wire previously calibrated is measured, its temperature 

 can be deduced. From theoretical considerations I showed that 



might be expected to represent the relation between the resistance 

 and absolute temperature. This formula agreed closely with my own 

 experimental results for platinum, copper, silver, iron, and aluminium 

 wires (" Journal of the Society of Telegraph Engineers and EleC-. 



